David Cameron was forced to admit yesterday that a British aid worker taken hostage in Afghanistan may have been killed by US special forces and not her captors as previously claimed.

Fresh evidence had come to light that Linda Norgrove, 36, could well have died as a result of a grenade thrown by soldiers as they tried to rescue her from Taliban captors on Friday, the prime minister said.

Cameron said earlier reports of how she had died were "deeply regrettable" but had been made in good faith.

Nato and British government sources told journalists on Sunday that Norgrove died when a hostage-taker detonated a bomb as Nato forces tried to free her.

The disclosure will raise difficult questions about the rescue operation and the way the media operation has disseminated the suggestion that Norgrove died at the hands of her captors.

After Cameron's statement, Nato was left denying suggestions that it had manipulated the truth. One source said senior Nato officials were angry that US forces had engaged in a "deliberate deception".

Norgrove was killed in the raid by US forces in Afghanistan's eastern Kunar province, after she and three colleagues were kidnapped two weeks earlier.

Downing Street said Cameron had spoken to US president Barack Obama who offered his condolences for Norgrove's death. "The prime minister and the president agreed that it was now essential to get to the bottom of what had happened in the course of the rescue operation," the statement said.

Earlier yesterday Cameron told a Downing Street press conference that he was told of the latest developments in a phone call from General David Petraeus, the top allied commander in Afghanistan. As a result, a full US-UK investigation has been launched into the incident, he said. Asked whether there was any suggestion US forces were reckless, the prime minister said: "We must get to the bottom of what happened; we only have sketchy information … Petraeus deeply regrets what happened, as do I."

The prime minister had to delay his scheduled press conference for an hour while he informed Norgrove's parents and younger sister of the "deeply distressing development".

So far, the family have declined to make any comment on their daughter's death and the operation to free her. John Norgrove, Linda's father, speaking from the family home in the Isle of Lewis, indicated he may speak at a later date.

Last nightit emerged that Norgrove's family had not been asked to approve the raid nor were told about it before it happened. It is understood that US forces' policy is to ask the hostage's family for prior approval before launching a rescue mission. British forces act without asking.

One British official said the Norgrove family had been told that a military rescue mission was possible but were not asked for their approval when the final decision was taken. "What we would never do is put the family in the situation where they are asked for a yes-no on going ahead with the action. That would put them in the invidious position of having to own the consequences of whatever decision was taken, which is properly the job of governments," the official said.

Surveillance footage of the rescue attempt, showing a US soldier throwing a grenade into the room where Norgrove was being held, led to yesterday's development according to the US military.

"The review showed what was believed to be a member of the rescue team throwing a grenade into the area where Miss Norgrove was being held," said Major Sunset Belinsky, a Nato spokeswoman.

But on Sunday, a Nato spokesmen said that an insurgent standing near Norgrove had "detonated an explosive device that was attached to his person" as American special forces dropped into the compound by helicopter. "He was in close enough proximity to Ms Norgrove. She was wounded," said spokesman Dominic Medley. "She was evacuated but succumbed to her wounds."

British defence sources said there was "very little UK military involvement" in planning the rescue attempt. In another official statement on her death, Hague told the House of Commons that he had authorised a military operation to rescue Norgrove "from the very beginning".

"All such rescue operations involve a measure of risk which has to be weighed against a constant risk to a hostage and a risk that such an opportunity to undertake a rescue operation may not recur," he said.

He said that British special forces liaised with their US counterparts, but did not play any role in planning or carrying out the rescue attempt.

Shadow foreign secretary Yvette Cooper condemned the actions of the hostage takers, but told MPs there was "concern" about the "potentially inaccurate information that was disseminated over the course of the weekend".

One former defence minister told the Guardian that the US-led investigation into Norgrove will be followed closely by the opposition. "They have shown that they cannot be fully trusted on this one," he said.

Sean Langan, an award-winning British documentary maker, was held hostage by Islamist extremists in Afghanistan for three months in 2008. He was released after successful negotiations with his kidnappers